If identity can be said to be real and impose limitations on the individual, my view would be that reason is the means to transcend it.

It's something we can control, or even dispose of altogether, if we want to, and if we develop the tools to understand it. — Tzeentch

Can we ‘want’ to want? Can we choose to will what we desire? This is not to suggest that what ‘drives’ reason is a fixed, pre-wired, unchanging mechanism. Rather, there is an interplay between reason and desire that shapes each in relation to the other. In sum , reason is not the slave of our passions , but neither are our drives the slave of reason.

Physically:

1. Internal reproductive organs.
The difference between men and women is quite clear: the former have testes that produce sperm and the latter have ovaries that produce eggs.

2. External genetalia.
Ambiguous as their development is hormonally regulated and diseases, exposure to certain chemicals can cause changes that result in mismatch between internal and external sexual organs.

Mentally:

There doesn't seem to be any patterns which we can identify as distinctly male or distinctly female; however, this is only if we look at it from a type perspective in re mental attributes; mayhaps there's a pattern if we give due consideration to degree. For instance, both dogs and humans are intelligent (indistinguishable), but humans are more intelligent than dogs (distinguishable). This means that we'll have to deal with grey areas (vagueness) on the spectrum, one end of which is full female and the other end of which is full male and by "deal with" I mean accept.